OBRABOTKAMETALLOV MATERIAL SCIENCE Vol. 24 No. 2 2022 a b Fig. 3. SEM image of the elements of the cross-section of the Cr5 coating in the back scattered electrons mode (a) and EDS distribution of elements in depth (b) tivity with iron and chromium melts under electric discharge conditions. Thus, in this case the mechanism of crystallization of ceramic phases from the melt after the terminated discharge is realized. Figure 3, a shows a cross-sectional image of the Cr5 coating and element distribution profi le data according to EDS analysis. The coating has a slightly darker shade compared to the substrate due to the enrichment with boron, which was not fi xed by the EDS analysis. Figure 3b shows a sharp transition between the deposited layer and the substrate. It also indicates a decrease in the concentration of substrate elements in the coating structure that is explained by the transfer of iron from the granules. The coating had a dense homogeneous structure with a small amount of small pores. With an increase in the powder concentration in the anode mixture, the average coating thickness decreased monotonically from 35.7 to 30.7 μm, and the roughness (Ra) increased from 7.1 to 9.1 μm (Table 1). Water contact angle (WCA) was measured to study the hydrophobic properties of the coating surface. The WCA is inversely proportional to the surface energy. As shown in Table 1, the WCA decreased from 70.2 to 57.6° with an increase in the concentration of CrB2 in the anode mixture that means decrease in the hydrophobicity of its surface. However, in general electrospark Fe-Cr-B coatings had lower surface energy and higher hydrophobicity compared to AISI 304 stainless steel (WCA 48.9°). Figure 4 shows the results of polarization testing of samples in 3.5% NaCl solution at room temperature. It shows that the potentiodynamic curves of all coatings have signifi cantly higher corrosion potential of Ecorr compared to AISI 304 steel. For detailed description of the corrosion behavior of the samples, the corrosion current Icorr was calculated from the slopes of the Tafel portions of the potentiodynamic curves (Table 3). It follows from Table 3 that with an increase in the amount of CrB2 powder in a mixture of granules, the corrosion current of the coatings monotonically decreased, which indicates an improvement in the anticorrosion behavior. Thus, saturation of the AISI 304 steel surface with chromium boride improves its anti-corrosion behavior. This is explained by the barrier action of a thin Cr2O3 fi lm inevitably formed on the surface of metallic chromium [25]. In addition, ceramic phases limit the contact area of the metal with the electrolyte [6]. Fig. 4. Tafel polarization curves of coatings and substrate
RkJQdWJsaXNoZXIy MTk0ODM1